[FIX] tools, mail: improve styling recognition in html fields

Purpose of this commit is to better support custom styling used notably
for mass mailing and outgoing emails. Indeed styling is whitelisted when
storing html. Finding it in html requires some parsing. This commit improves
the regex used to find it when semicolons are involved. It allows more use
cases to be correctly supported.

Task ID #2125856
PR #40433

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>